Fe Isotope Fractionation Research Articles

Application of Fe isotopes to tracing the geochemical and biological cycling of Fe

Over 100 high-precision Fe isotope analyses of rocks and minerals are now available, which constrain the range in δ 56Fe values (per mil deviations in 56Fe/ 54Fe ratios) in nature from −2.50‰ to +1.5‰. Re-assessment of the range of δ 56Fe values for igneous rocks, using new ultra-high-precision analytical methods discussed here, indicate that igneous Fe is isotopically homogeneous to ±0.05‰, which represents an unparalleled baseline with which to interpret Fe isotope variations in nature. All of the isotopic variability in nature lies in fluids, rocks, and minerals that formed at low temperature. Equilibrium (“abiotic”) isotopic fractionations at low temperatures may explain the range in δ 56Fe values; experimental measurements indicate that there is a large isotopic fractionation between aqueous Fe(III) and Fe(II) ( Δ Fe(III)–Fe(II)=2.75‰). However, many of the natural samples that have been analyzed have an unquestionable biologic component to their genesis, and the range in δ 56Fe values are also consistent with the experimentally measured isotopic fractionations produced by Fe-reducing bacteria. In this work, we touch on a number of aspects of Fe isotope geochemistry that bear on its application to geochemical problems in general, and biological cycling of metals in particular. We report on new state-of-the-art Fe isotope analytical procedures, which allow precisions of ±0.05‰ ( 56Fe/ 54Fe) on samples <300 ng in size. In addition, we discuss the implications of experimental work on Fe isotope fractionations during metabolic processing of Fe by bacteria and the need to take a “mechanistic” approach to understanding the pathways in which Fe isotopes may be uniquely fractionated by biology. Additionally, we discuss experimental methods, such as the use of enriched isotope tracers that are necessary to evaluate if experimental isotope exchange reactions are transient kinetic fractionations, equilibrium isotopic exchange reactions, or a combination of both, which can be caused by the complexities of multiple isotope exchange reactions taking place in an experimental system.

Kinetic and equilibrium Fe isotope fractionation between aqueous Fe(III) and hematite

Application of the Fe isotope system to studies of natural rocks and fluids requires precise knowledge of equilibrium Fe isotope fractionation factors among various aqueous Fe species and minerals. These are difficult to obtain at the low temperatures at which Fe isotope fractionation is expected to be largest and requires careful distinction between kinetic and equilibrium isotope effects. A detailed investigation of Fe isotope fractionation between [Fe III(H 2O) 6] 3+ and hematite at 98°C allows the equilibrium 56Fe/ 54Fe fractionation to be inferred, which we estimate at 10 3lnα Fe(III)-hematite = −0.10 ± 0.20‰. We also infer that the slope of Fe(III)-hematite fractionation is modest relative to 10 6/T 2, which would imply that this fractionation remains close to zero at lower temperatures. These results indicate that Fe isotope compositions of hematite may closely approximate those of the fluids from which they precipitated if equilibrium isotopic fractionation is assumed, allowing inference of δ 56Fe values of ancient fluids from the rock record. The equilibrium Fe(III)-hematite fractionation factor determined in this study is significantly smaller than that obtained from the reduced partition function ratios calculated for [Fe III(H 2O) 6] 3+ and hematite based on vibrational frequencies and Mössbauer shifts by Polyakov (1997), Polyakov and Mineev (2000), and Schauble et al. (2001), highlighting the importance of experimental calibration of Fe isotope fractionation factors. In contrast to the long-term (up to 203 d) experiments, short-term experiments indicate that kinetic isotope effects dominate during rapid precipitation of ferric oxides. Precipitation of hematite over ∼12 h produces a kinetic isotope fractionation where 10 3lnα Fe(III)-hematite = +1.32 ± 0.12‰. Precipitation under nonequilibrium conditions, however, can be recognized through stepwise dissolution in concentrated acids. As expected, our results demonstrate that dissolution by itself does not measurably fractionate Fe isotopes.

Mass-dependent fractionation of Mg, Si, and Fe isotopes in five stony cosmic spherules

We have measured with an electron microprobe the Mg, Al, Si, Ca, Ti, Mn, and Fe contents of five strongly heated stony cosmic spherules (sCS) from the South Pole water well. We have also measured the isotopic compositions of Si, and when possible of Mg and of Fe in these objects by ion microprobe. Except for iron, the measured elemental compositions are chondritic within a factor of 2. In four samples, the ratio of 57Fe/56Fe exceeds the terrestrial value by 3.5‰ to 48‰. Mass-dependent fractionation of the isotopes of Si ranges from ∼2 to ∼8 ‰/AMU in three samples. Mg is clearly fractionated in only one sample, for which δ25Mg = ∼8 ‰. The extent of mass-dependent fractionation of the isotopes and, by implication, of evaporative loss generally follows a trend Mg < Si < Fe. The trend is similar to that found in laboratory heating experiments of charges with solar composition. Although the observed isotopic inhomogeneities within some samples call into question the strict validity of the Rayleigh equation for the sCS, its approximate application to our new and to previously published results for Mg suggests that evaporative losses of greater than 40 wt.% occur rarely from sCS, and that the precursor grains of the sCS had a CM-carbonaceous-chondrite-like complement of Mg, Si, Ca, and Al. Low Fe contents relative to CM abundances could reflect an unusual precursor composition, or, more probably, losses by processes that did not fractionate isotopes, i.e., ejection of immiscible FeS and FeNi beads from the melt or rapid, complete separation and decomposition of FeS at the surface.

Isotopic fractionation between Fe(III) and Fe(II) in aqueous solutions

Large equilibrium isotope fractionation occurs between Fe(III) and Fe(II) in very dilute (≤22 mM Cl −) aqueous solutions, reflecting significant differences in bonding environments. Separation of Fe(III) and Fe(II) is attained by rapid and complete precipitation of Fe(III) through carbonate addition, followed by separation of supernatant and ferric precipitate; experiments reported here produce an equilibrium Δ Fe(III)–Fe(II)=+2.75±0.15‰ for 56Fe/ 54Fe at room temperature (22±2°C). The timescales required for attainment of isotopic equilibrium have been determined by parallel isotope tracer experiments using 57Fe-enriched iron, which are best fitted by a second-order rate law, with K=0.18±0.03 s −1. Based on this rate constant, ∼15–20% isotopic exchange is estimated to have occurred during Fe(III)–Fe(II) separation, which contributes <0.10‰ uncertainty to the equilibrium Δ Fe(III)–Fe(II). Under the experimental conditions used in this study, >97% Fe(II) exists as [Fe II(H 2O) 6] 2+, and >82% Fe(III) exists as [Fe III(H 2O) 6] 3+ and [Fe III(H 2O) 6− n (OH) n] 3− n ; assuming these are the dominant species, the measured Fe isotope fractionation is approximately half that predicted by Schauble et al. [Geochim. Cosmochim. Acta 65 (2001) 2487–2497] at 20–25°C. Although this discrepancy may be due in part to the experimentally unknown isotopic effects of chloride interacting with Fe-hexaquo or Fe-hydroxide complexes, or directly bonded to Fe, there still appears to be at this stage a >1‰ difference between prediction and experiment.

Iron isotopes in hot springs along the Juan de Fuca Ridge

We present Fe isotope data for high temperature hydrothermal fluids, sulfides, and basalts from the Juan de Fuca Ridge, USGS basalt standard BCR-2 and the Canyon Diablo iron meteorite. The Fe isotopic compositions of all samples fall within a range of ∼1‰. However, there is a small but statistically significant isotopic variation between Fe dissolved in hydrothermal fluids and Fe in Canyon Diablo. Specifically, the hydrothermal fluids are lighter compared to Canyon Diablo, yielding δ56Fe (=[(56Fe/54Fe)sample/(56Fe/54Fe)Canyon Diablo−1]×1000) values from −0.30‰ to −0.77‰. In contrast, the BCR-2 basalt and Canyon Diablo are isotopically indistinguishable. Measurements of δ56Fe in Juan de Fuca Ridge basalts and sulfides suggest that Fe isotopes are fractionated in hydrothermal systems. However, the data do not exclude the possibility of small δ56Fe differences between the rocks from which fluid Fe is derived and other igneous Fe, in which case fractionation in igneous systems is indicated. Regardless, these results are evidence of non-biological mass fractionation of Fe isotopes in nature. Further, venting of hydrothermal fluids into the overlying water column could provide a source of isotopically light Fe to the deep oceans, contributing to Fe isotopic variations observed in deep-sea sediments.

Iron isotopes in chondrules: Implications for the role of evaporation during chondrule formation

Abstract— We have measured the δ57Fe of olivines in nine Chainpur chondrules. All are within error of normal (typically 2σ ≤ 1–2%0). Most of the chondules could not have lost more than ∼20% of their FeO by Rayleigh evaporation and none can have lost more than ∼61%. Yet, the range of Fo contents in these chondrules is Fo78–99.9. The isotopic compositions of the chondrules clearly demonstrate that, for instance, type I chondrules cannot form from type II chondrules by evaporation of FeO under Rayleigh conditions. The isotopic compositions also place constraints on the minimum cooling rates these chondrules could have experienced. These cooling rates must also be equal to or slower than those required to produce the chondrule textures. Assuming flash heating and evaporation rates like those measured in vacuum, the minimum cooling rates necessary to prevent detectable Fe isotopic fractionation via Rayleigh evaporation approach those needed to produce barred and porphyritic textures. The presence of hydrogen in the nebula, non‐linear cooling and other effects will all tend to increase the cooling rates required to prevent δ57Fe &gt; 1–2%0, perhaps by as much as 1–2 orders of magnitude. The two most likely ways that the cooling rates required to prevent δ57Fe &gt;1–2%0 can be kept below those needed to produce barred and porphyritic textures are (1) the pH2 in the nebula was low enough to keep evaporation rates close to those in vacuum, or (2) back reaction of chondrules with Fe in the gas suppressed isotopic fractionation.

Fractionation of Fe isotopes by soil microbes and organic acids

Research Article| June 01, 2001 Fractionation of Fe isotopes by soil microbes and organic acids Susan L. Brantley; Susan L. Brantley 1Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar Laura Liermann; Laura Liermann 1Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar Thomas D. Bullen Thomas D. Bullen 2U.S. Geological Survey, Menlo Park, California 94025, USA Search for other works by this author on: GSW Google Scholar Geology (2001) 29 (6): 535–538. https://doi.org/10.1130/0091-7613(2001)029<0535:FOFIBS>2.0.CO;2 Article history received: 05 Sep 2000 rev-recd: 26 Feb 2001 accepted: 06 Mar 2001 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Susan L. Brantley, Laura Liermann, Thomas D. Bullen; Fractionation of Fe isotopes by soil microbes and organic acids. Geology 2001;; 29 (6): 535–538. doi: https://doi.org/10.1130/0091-7613(2001)029<0535:FOFIBS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Small natural variations in Fe isotopes have been attributed to biological cycling. However, without understanding the mechanisms of fractionation, it is impossible to interpret such variations. Here we show that the δ56Fe of Fe dissolved from a silicate soil mineral by siderophore-producing bacteria is as much as 0.8‰ lighter than bulk Fe in the mineral. A smaller isotopic shift is observed for Fe released abiotically by two chelates, and the magnitude of the shift increases with affinity of the ligand for Fe, consistent with a kinetic isotope effect during hydrolysis of Fe at the mineral surface. Fe dissolved abiotically without chelates shows no isotopic shift. The δ56Fe of the exchange fraction on soil grains is also lighter by ∼0.6‰–1‰ than Fe from both hornblende and iron oxyhydroxides. The kinetic isotope effect is therefore preserved in open systems such as soils. When recorded in the rock record, Fe isotopic fractionation could document Fe transport by organic molecules or by microbes where such entities were present in the geologic past. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Demonstration of significant abiotic iron isotope fractionation in nature

Field and laboratory studies reveal that the mineral ferrihydrite, formed as a result of abiotic oxidation of aqueous ferrous to ferric Fe, contains Fe that is isotopically heavy relative to coexisting aqueous Fe. Because the electron transfer step of the oxidation process at pH >5 is essentially irreversible and should favor the lighter Fe isotopes in the ferric iron product, this result suggests that relatively heavy Fe isotopes are preferentially partitioned into the readily oxidized Fe(II)(OH) x (aq) species or their transition complexes prior to oxidation. The apparent Fe isotope fractionation factor, α ferrihydrite- water , depends primarily on the relative abundances of the Fe(II) (aq) species. This study demonstrates that abiotic processes can fractionate the Fe isotopes to the same extent as biotic processes, and thus Fe isotopes on their own do not provide an effective biosignature.

In-situ isotopic ratio analysis of iron using laser ablation-multiple collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)

In-situ 54Fe/56Fe and 57Fe/56Fe ratio measurements for solid samples have been carried out using laser ablation-multiple collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). Using a dry plasma condition achieved by laser sample introduction technique, mass spectrometric interferences such as 54ArN+, 56ArO+ or 57ArOH+ could be successfully reduced. Although these argide interference signals were not completely eliminated, the contribution of the remnant signals could be effectively corrected by an “on-peak” baseline subtraction technique. Resultant repeatability or external precision of the measurement over the four month period was almost 0.5–1%, which was insufficient to detect the isotopic variations of Fe in natural samples (typically smaller than 0.3%). We found that the poor repeatability in Fe isotopic ratio measurements was mainly due to instability or poor reproducibility of the high voltage power supply, especially for ion acceleration voltage, of our instrument. The mass discrimination effect on Fe isotopic ratios was strongly correlated with accelerating energy (ion energy) of analyte ions. To overcome this, Fe isotopic ratios of NIST Standard Reference Material (SRM) 665 metallic standard were measured between the sample measurements. This sample–standard bracketing method successfully results in internal precisions of the 54Fe/56Fe and 57Fe/56Fe ratios of better than 0.1% at the 95% confidence level. The analysis time for one sample was about 50 s, and the total amount of Fe used for an analysis was about 30 ng. The 54Fe/56Fe and 57Fe/56Fe ratios measured for NIST SRM 661 steel, three Fe-bearing minerals, and six iron meteorites, show no significant variations within the analytical uncertainties achieved in this study. The good agreement between samples suggests that our approach successfully compensates for both the instrumental drift and the background correction. The data presented here demonstrate clearly that the Fe isotopic ratios obtained by the LA-MC-ICP-MS technique can give strong clues to elucidate the Fe isotopic fractionation occurring in biochemical reactions, which may cause a larger isotopic fractionation effect in nature.